Add fallback to French (fr) from fr-CA locale

Setting browser display language to French (Canada) 'fr-CA' locale results in Nuxeo labels displaying in English, rather than expected French 'fr' locale.

Steps to Reproduce:

1. From Browser settings, set display language to French (Canada) and Relaunch.
2. Navigate to Web UI and observe the label display language.

Expected behavior: labels displayed fall back to French (defined in messages-fr.json)
Actual behavior: labels displayed show in English (defined in messages.json)